macintosh/adb: make adb_dev_class constant

Since commit 43a7206b ("driver core: class: make class_register()
take a const *"), the driver core allows for struct class to be in
read-only memory, so move the adb_dev_class structure to be declared
at build time placing it into read-only memory, instead of having to
be dynamically allocated at boot time.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240305-macintosh-v1-1-9c3f4f882045@marliere.net